1. Field of the Invention
The present invention relates to methods and apparatus for carrying out primary cementing operations in a well bore without incurring substantial gas inflow.
2. Description of the Prior Art
In the primary cementing of wells, a pipe disposed in a well bore, e.g., casing, is cemented therein whereby the pipe is bonded to the walls of the well bore. The bonding serves to maintain the pipe in place and to prevent subterranean formation fluids from communicating between zones or to the surface by way of the annulus between the well bore and the pipe. Typically, primary cementing is carried out by pumping a cement slurry downwardly through the pipe to the bottom thereof and then upwardly into the annulus. The cement slurry is displaced out of the pipe and into the annulus by a displacement fluid and upon being placed in the annulus, the cement slurry is allowed to set into a hard impermeable mass therein.
When one or more of the subterranean formations or zones penetrated by the well bore contain pressurized gas, a problem often encountered involves the flow of gas into the cement slurry and into the well bore prior to when the cement slurry sets. Such gas can flow to the surface, create communication between producing or other subterranean formations or zones and can, when in high enough volume, create a blow out shortly after the primary cementing operation is complete.
The occurrence of gas flow in a cemented pipe-well bore annulus relates to the inability of the cement slurry to transmit hydrostatic pressure during the transition period in which the slurry converts from a true fluid to a partially self supporting semisolid as a result of the development of static gel strength. Static gel strength can be defined as internal rigidity in the matrix of the cement that resists a force placed upon it. The development of static gel strength starts immediately after placement of the cement slurry in the annulus and continues to increase as the cement hydrates or sets. At a point before initial set, the cement slurry develops a static gel strength high enough to prevent pressurized gas from moving through it.
When the cement slurry is initially placed in the annulus, it exerts hydrostatic pressure on gas-containing formations penetrated by the well bore whereby gas flow into the cement slurry and into the well bore is prevented. However, after the cement slurry becomes partially self-supporting, the ongoing hydration reactions in the slurry and fluid losses therefrom cause volume reductions to take place in the slurry which in turn causes the hydrostatic pressure exerted by the slurry to be reduced. When the hydrostatic pressure is reduced below the pressure of the gas, gas flow through the slurry contained in the well bore takes place.
A number of methods and special hydraulic cement compositions have been developed and used to minimize gas inflow during primary cementing. Examples include multiple stage cementing methods, maintaining a back pressure on the annulus and/or including special additives in the cement composition such as latexes, special fluid loss prevention additives, in situ gas generating additives, etc. While the prior methods and cement compositions have achieved varying degrees of success, they are relatively expensive and problems caused by gas inflow still often result.
Thus, there is a need for improved methods and apparatus for carrying out primary cementing operations in wells whereby expensive cement slurry additives are not required and gas flow into the cement slurry and well bore is substantially prevented.